High-power, high-energy electron beam guns require special window materials for their operation. For many high-pressure electron-gun (e-gun) applications the maximum pressure differential that the electron beam window can withstand limits its range of applicability. Many high strength materials which might be used for electron beam windows suffer from either one of two disadvantages (1) they are high-Z foils and thus have large electron absorption cross sections or (2) they contain elements as alloy constituents which over time cause cathode degradation.
Of particular interest is the widespread use of a high-power, high-energy electron beam gun in high-power electric discharge lasers. The windows for this use have attracted the interests relating to improvements needed.
The use of electron beam ionization in high-power electric discharge lasers is currently widespread but improvements to electron beam windows are needed. This method of ionization is an attractive means for producing large-volume ionization in molecular gases. Present day electron beam sustained lasers require electron beams with uniform current densities over large beam areas. Generally, these lasers operate in either pulsed or continuous modes with peak pressures in the laser channel of several atmospheres. The electron beam gun which serves as the source of electrons to produce ionization in the laser channel operates in a vacuum environment. Thus, a vacuum interface is required to isolate the gun environment from the higher-pressure laser cavity environment. This interface, the electron beam window, is generally comprised of a thin foil material having high electron transmission over the useful energy range of the gun. Poor reliability of these windows has been one of the problems which has limited the usefulness of electron beam guns in laser applications. In wide-area electron beam guns the foil window bears on a foil support structure which provides both mechanical support and cooling for the window.
There are several factors which make the design and fabrication of a reliable electron beam window for laser applications difficult. Cooling requirements for high current levels are stringent. It is desired to operate the guns at conditions which result in volumetric heat deposition rates in the window material of 10 to 50 kw/cm.sup.3 of foil material. The window must be thin and must be made of low atomic number material to minimize absorption of electrons. The window must be vacuum tight and capable of withstanding pressure differentials as high as 10 atm in some systems. The window must also be able to withstand transient stress loading arising from acoustic pulses produced in the laser channel during short pulse operation.
The investigations of the needed requirement have provided information necessary to develop a reliable, 15-cm-wide by 100-cm-long, cooled electron beam window for pulsed or continuous operation. The basic design point conditions are: (1) maximum mean laser operating pressure of 2 atm with a pulsed pressure of up to 4 atm; (2) in pulsed operation, maximum current density transmitted through the window of 10 mA/cm.sup.2 throughout the range of beam energies between 100 and 200 keV; (3) pulse repetition frequency of 100 Hz; or if possible up to 1000 Hz; and (4) in continuous operation, maximum current density transmission of 0.3 mA/cm.sup.2, and if possible up to 1.5 mA/cm.sup.2 for short durations. At the time this program was initiated, attainment of these continuous current densities was beyond the state of the art of available electron beam hardware.
The investigations were centered around three parts: (1) evaluation of foil window material characteristics, (2) evaluation of various window support configurations and geometries to provide both support and cooling of the thin foil window, and (3) small-scale testing of promising support configurations and foil materials in continuous and pulsed operation, with and without heat addition to the foil.
Other investigations considered beryllium foil windows because of its low atomic number. Disadvantages for use of the beryllium foil included high cost and poor availability. For example, it is estimated that a 15 cm wide by 100 cm long beryllium foil window, 25 .mu.m thick, would have a material cost of approximately $5300, whereas for aluminum only approximately 2 cents worth of commercial material is required.
Carbon was also considered as a candidate foil material for electron beam windows since the technology for producing thin carbon tape does exist for producing a suitable foil of small size. Carbon is low atomic number (6) and its high-temperature properties offset somewhat its low thermal conductivity and thermal diffusivity. The problem of fabricating sufficiently large samples of carbon foil having the ductility required will require additional research.
An electron beam window constructed of a composite was considered to have potential merit; therefore, additional research was devoted to this approach for developing an electron beam window that would be particularly attractive for use in combination with a high-power, high-energy electron beam gun.
The advantages of employing a window constructed of a composite material would be to select and utilize the characteristics of each material that will accrue the desired benefits for an improved electron beam window.
Therefore, an object of this invention is to provide a composite window for use with a high-power, high-energy electron beam gun which will permit a wider range of use for the high-power, high-energy electron beam gun.
A further object of this invention is to provide a window for use with a high-power, high-energy electron beam gun wherein the window is constructed of composite material with one of the materials having good heat transfer characteristics for removing a greater portion of the energy deposited in the material by the electron beam and the other material having a high strength and suitable conductance properties for conducting additional heat energy for dissipation by the material having the good heat transfer characteristics.